Technical Abstract:
Agricultural drainage ditches are important routes of preferential flow in agricultural systems providing a rapid, more direct path for nutrient (N and P) laden drainage waters to reach downstream environments. Drainage ditches are forgotten links between agricultural farms and receiving waters, thus, very little research has gone into investigating the utilization of agricultural landscape features for mitigation of nutrients associated with drainage and stormflows. We undertook a two year study of two drainage ditches under no-till cotton to monitor nutrient flux on both spatial and temporal gradients. Water levels within ditches provided storm and baseflow hydrographs that combined with flow concentrations yielded nutrient loads. Water samples were collected monthly, and with every storm event that generated surface runoff and elevated water levels within the ditch. For baseflows, ditches were sources of nitrate and ammonium throughout the growing and dormant seasons, while in comparison, ditches were sinks for dissolved reactive phosphorus (DRP) and particulate phosphorus (PP) for growing and dormant seasons. Monthly nitrogen and phosphorus stormflows in ditches were highly variable between sink and source, and were largely dependent on hydrological variability. Agricultural drainage ditches retained 22-64% of nitrate and ammonium and 31-60% of DRP and PP stormflow nutrient loads over the two year study period. Inter-annual hydrological variability plays and important, engaging role in the flux of phosphorus and nitrogen species, nutrient mitigation capacity of agricultural drainage ditches and nutrient load contamination of downstream environments.